Catsden,
If you haven't transferred possession to the customer yet, I would count it as an "advance" and still enter it & keep the ribbon. Especially since she told you you could keep it until the competition, I don't think you'll have any issues. Be sure to carefully document any prize and pass along that paperwork/documentation to the client in case she wants to get her quilt appraised.
Perhaps you could also contact the show coordinator & ask if they are able to provide an additional ribbon for the quilt owner in the event a client's quilt wins (might be a small extra charge for that if they do -- but others have them specially printed & aren't able to accommodate requests for extra ribbons so it just depends).
If you're still concerned after that, I would address it directly with the client. If you've never won a ribbon before, let her know that & apologize for not discussing how to handle this earlier. Then offer some options to her to help document the win (if you win -- hope you do!) -- details about show so she can come see it & take pictures of/with it at the show, program if it lists what quilts were chosen or a letter from the committee if it's a curated show to demonstrate that your/her quilt was selected to competition, maybe a photo of both of you receiving the ribbon together, and any certificate(s) that you get showing the win (could give her the original & keep a copy for yourself -- or vice versa). I really think most clients are quite reasonable about things like this, but not all of them. If I ever go back to doing commissioned quilts again (one too many requests to do things that weren't making me happy -- so now I just make what I love & then find a seller), I'm really getting to the point that I would put on my order form that any proceeds from the quilt that occur prior to transfer of possession will be retained by me and transferred at my sole personal discretion. I've been really lucky to have wonderful clients but all this monkey business I hear about online and from quilting colleagues in real life just makes the need for a well-crafted agreement at the time of initial deposit seem all the more essential.